An endoscope is inserted into narrow spaces such as human internal body parts or the interior of an engine. Therefore it is desirable that the outer diameter of an endoscope be as small as possible, and this requires that the diameter of the lens that is arranged at the tip of an endoscope be as small as possible. An example of a lens of an endoscope that has a comparatively small diameter and wherein the lens is made using only a few lens elements is described in Japanese Laid-Open Patent Applications H9-68647 and H7-84179.
FIG. 9 illustrates the same objective lens element structure as that of Example 1 of Japanese Laid-Open Patent Application H9-68647, but includes different labels and, in addition to showing the objective lens elements themselves, FIG. 9 also illustrates conjectured support structure and flare-reducing structure for these lens elements. The lens comprises, in order from the object side, a first lens element lens L1, an aperture stop 5 (ie., a brightness diaphragm), a second lens element L2 of positive refractive power and a third lens element L3 of positive refractive power. In one embodiment of Japanese Laid-Open Patent Application H9-68647, lens element L1, which in all other embodiments is piano-concave, is replaced by a plane-parallel cover plate. Although no support structure whatsoever for these lens elements is actually shown in Japanese Laid-Open Patent Application H9-68647, FIG. 9 illustrates the assumed support structure for these components based on standard practices in the art, as will now be discussed. Because the first lens element L1 (or cover plate, as the case may be) and the second lens element L2 have the same outer diameter, they have been illustrated in FIG. 9 as being fixed to a first support frame SF1. An aperture stop 5 lies between the first lens element L1 and the second lens element L2. Thus, the first support frame SF1, is shown supporting these three elements. Further, it is likely that the outer diameter of the first support frame SF1 would be sized to equal the outer diameter of the third lens element L3, which in Japanese Laid-Open Patent Application H9-68647 is shown as having an outer diameter that is slightly larger than the outer diameter of the other two lens elements. Having their outer diameters equal would enable the first support frame SF1 and the second lens element L2 to each be inserted into a second support frame SF2, as illustrated in FIG. 9. Adjacent lens element L3 in FIG. 9 is illustrated a conjectured flare diaphragm 7 and a diaphragm receiving member 8, as would commonly be used in the prior art to form a flare stop.
FIG. 10 illustrates the lens element structure of Example 6 of Japanese Laid Open Patent Application H7-84179. The lens of this example includes, in order from the object side, a plano-concave lens L1, an aperture stop 5, a positive meniscus lens element L2, and a positive lens element L3.
In image formation lenses generally, in order to remove the light which does not contribute to image formation, a flare stop is often provided. There is no known description in the above-mentioned two patents which relates to a flare stop. However, as shown in prior art FIG. 9 for optical diaphragms generally, a flare diaphragm 7 and a diaphragm receiving member 8 are often provided on the object side of lens element L3. In many cases, such a flare stop is provided at this position in order to prevent stray light that has been reflected off the inner surface of the frame from passing through to the observer. The flare stop is positioned before lens element L3 because stray light rays (i.e., those from a larger angle of view than for which the lens was designed to image) in many cases are incident on the frame between positive lens elements L2 and L3.
However, if the outer diameter of a lens element is made smaller to achieve size reduction, the associated diaphragm receiving member and flare stop member must also be made smaller. Because these members are each shaped as an annular ring which circumscribes an aperture having a diameter that is nearly the same diameter as the lens element itself, as the lens element becomes smaller, the width of the annular ring becomes so small that manufacture and assembly of the ring become difficult.
Although it is known to bevel the edge of a lens element so as to form a sloped surface near the circumference of the lens element and to use this surface to self-center a lens element in lieu of using a spacer ring (as taught, for example, in U.S. Pat. No. 5,589,989) there is no teaching in this reference or any other reference known to applicant which suggests beveling an edge of a lens element and applying a light shielding material (i.e., one which absorbs or attenuates light) to a sloped surface created by the beveling process in order to reduce flare. Nor is there any suggestion that such an approach to reducing flare will result in fewer components and result in an overall cost reduction for assembling a lens system having small diameter components.